Flame retarding acrylonitrile polymer compositions



Unite States Patent 3,376,253 FLAME RETARDING ACRYLGNITRILE POLYMERCOMPOSITIONS Edward V. Buruthall and Julian I. Hirshfeld, Decatur, Ala,assignors to Monsanto (lampany, St. Louis, M0., a corporation ofDelaware N0 Drawing. Filed May 5, 1966, Ser. No. 547,742 12 Claims. (Cl.260-457) ABSTRACT OF THE DISCLOSURE The instant invention relates to anovel method of producing flame retardant synthetic compositions and tothe novel compositions so produced. More particularly, the instantinvention provides a novel method for producing acrylic fibers and filmshaving a high degree of permanent flame resistance, and to the novelacrylic articles themselves which remain flame resistant over longperiods of time despite use and laundering.

Flame and heat resistant fabrics in general have been commonplace formany years. One method of achieving flame resistance in fibers and filmshas been to introduce a chlorine or phosphorus-containing monomer in thesynthetic polymer chain. Although such specialty polymers are useful inspecific applications, in general such polymers do not possess theall-around qualities of dyeability, resiliency, texture and hand whichis demanded of a commercial synthetic fiber to be used in consumertextile applications. Moreover, in many of these applications, althoughflame resistance would be desirable from safety and other standpoints,it is not mandatory and hence the disadvantages of specialty polymershave essentially precluded them from this market.

Another approach to flame resistance has been to impregnate the fiberwith a flame resistant composition, as for example by immersion,spraying or the like. Such techniques suffer from two primedisadvantages. Initially, such treatment usually adversely affects theappearance and hand of the fiber and hence is not usually desirable, andsecondly, the degree of flame resistance available by such techniques isgenerally only temporary inasmuch as, at least in the case of fibers,the flameproofing agent is usually gradually removed as a result of wearand laundering.

Fluoboric acid and the metal fluoborates have been recognized in thepast as having beneficial flame retardant properties and accordinglyhave been applied to fabrics in the past with an eye towardflameproofing. However, fluoboric acid itself is extremely soluble inwater, and likewise are many of the metal fiuoborates, such as sodiumfluoborate and the like. Accordingly, despite the thoroughness withwhich the fiber is impregnated with these compounds, eventuallylaundering, particularly at high temperatures, leaches the fluoboricacid or fluoborate from the fiber, thus eliminating the flame resistanteffect. Attempts to substitute insoluble or sparingly solublefluoborates as flameproofing agents have been equally unsuccessfulinasmuch as these compounds must be applied to fabrics from a dispersionin water. Hence, the application is extremely diificult and results onlyin a surface 3,376,253 Patented Apr. 2, 1968 coating of flameproofingagent which usually adversely alfects the texture and hand of thefabric, and is invariably at least partially removed during the use ofthe treated article.

The difiiculties in fabric texture which arise as the result offlameproofing treatment consequently provoke further countermeasures inan attempt to restore fabric resiliency and hand. Quite often, theflameproofing of fabrics is accompanied by the addition of plasticizersand like compounds to the fabric in order to combat the harshnessusually imparted by the flameproofing treatment. Thus, despite theapparent desirability of flame retardant properties in everyday fabrics,flameproofing is not usually available in fabrics where such arequirement is not demanded because of a special end use due to thenumerous measures and countermeasures which must be undertaken tofinally produce a fabric of satisfactory quality.

It is accordingly an object of this invention to provide a novel processfor spinning flame resistant acrylic fibers which possess a pleasingtexture and hand and which at once retain their flame retardantproperties after prolonged use and laundering. It is a further object ofthis invention to provide a process for producing flame retardantacrylic fibers which are impregnated with a metal fluoborate which isnot leached from the fiber during washing, even at high temperatures. Itis a further object of this invention to provide such novel acrylicfibers so impregnated. It is another object of this invention to providea novel wet spinning technique for producing acrylic fibers which do notsupport a flame. Although the instant specification will be primarilydirected to the production of flame resistant fibers from acrylicpolymers, it is to be understood that the instant novel process isequally eflicient for producing novel acrylic films and other articleswhich are extruded by wet extrusion techniques, i.e., those articlesproduced by extruding an acrylic dope solution into a coagulant.

These and other objects are achieved by the novel process of the instantinvention wherein acrylonitrile polymer is dissolved in fluoboric acidand then coagulated as a filament or a film in a coagulating bathcontaining metal salts, the metal portions of which form insolublefluoborates upon reaction with fluoboric acid. Accordingly, there isformed in situ within the fiber fluoborate salts which are relativelyinsoluble in water and hence which are not subject to leaching duringsubsequent laundering of the fiber. Furthermore, inasmuch as theseinsoluble fluoborates are formed within the fiber at a point when thefiber is still in a gelatinous state the uniformity of dispersion of thefluoborates through the fiber is remarkably uniform. Furthermore, the insitu formation of these fluoborate salts result in the thoroughimpregnation of the fiber with a flameproofing agent without adverselyaffecting the hand or texture of the ultimate fiber.

In the practice of the instant invention the acrylonitrile polymer dopeis prepared in much the same manner as is employed in ordinary Wetextrusion techniques. Initially, the polymer in a finely divided solidstate is mixed with the fluoboric acid to form a dope solution. Thesolution may be heated for example from 25 up to C. to raid dissolution.Generally the acrylonitrile polymer will be dissolved in the fluoboricacid in such amount as to provide from about 5 to about 35 percent byweight of polymer in the solvent. Of course, optimum values ofconcentration will be determined by the molecular weight of theparticular polymer being spun, and accordingly the ultimate viscosity ofthe polymer solution. Although the low concentrations of polymer insolvent can be used in extrusion operations, such concentrations areundesirable because they necessitate the removal and recovery of toomuch solvent from the extruded solution, thereby increasing solventrecovery cost and reducing spinning speeds by reason of the longerperiod required for coagulation. Particularly in the instance of thepresent invention, the expense of fiuoboric acid necessitates itsrecovery from the coagulating bath, and accordingly the concentration ofpolymer in the fluoboric acid solution is preferably maintained betweenabout 10 and 25 percent.

Fluoboric acid will function adequately as a solvent for acrylonitrilepolymer at aqueous concentrations ranging from 35 percent to about 60percent by weight. Typically, for example, a 48 percent solution offluoboric acid in water will perform satisfactorily as a solvent for theacrylic polymer, though higher and lower concentrations may be used.

Fluoboric acid need not be the sole polymer solvent used in preparingthe dope solution. To minimize the expense and the need for recovery, itis preferred that the acrylic polymer solution be prepared using acosolvent system comprising fluoboric acid and a miscible compatiblepolyacrylonitrile solvent such as dimethylformamide ordimethylacetamide. Generally the fiuoboric acid should be present insuch amount as to provide at least about percent by weight based uponthe Weight of the acrylic polymer in the dope solution. However, theamount of fluoboric acid employed in the dope system will be highlydependent upon the ultimate desired concentration of fluoborate in thecoagulated fiber. Hereinafter, reference to the fiuobon'c acid solutionof the acrylonitrile polymer is deemed to include solutions whereinfluoboric acid is employed as a cosolvent with a second miscible andcompatible acrylonitrile polymer solvent.

The fibers are spun by extruding the fluoboric acid so lution of theacrylonitrile polymer through an orifice or a spinneret having aplurality of orifices into a liquid medium which at once results in theformation of insoluble or sparingly soluble fluoborates in situ in thefiber and also extracts from the gelatinous fiber the remainder of thepolymer solvent. In accordance with this invention the coagulating bathcontains dissolved amounts of metal salts, the metal portion of whichwill combine with the fluoboric acid to form sparingly solublefiuoborates in situ within the fiber. The metals, potassium, rubidium,and cesium have been found to be effective for this purpose, andaccordingly a salt of one or more of these metals must be present in thecoagulating bath to effect the proper in situ formation of the insolublefluoborate fiameproofing agents. A liquid medium employed in the coagulating bath is not critical so long as it functions properly to removethe solvent from the filaments and is capable of dissolving the metalsalt used in the formation of the in situ insoluble fluoborates. Typicalof liquidmediums which are useful for this purpose are simple aqueoussolutions of soluble potassium salts. When an organic cosolvent is used,the coagulating bath may contain up to about 65% of the organic solvent.

As stated above, the potassium, rubidium and cesium salts of thefluoborates are sufficiently water insoluble to function properly aspermanent fiameproofing agents in accordance with the instant invention.

In this regard, the terms insoluble and sparingly soluble as used hereinrefer to fluoborate salts having a maximum solubility in water of aboutone-half mole per liter. In general, any of these metals may beintroduced into the coagulating bath solution as any suitable solublesalt otherwise inert to the system. Thus, suitable salts of the abovemetals include the phosphates, sulfates, halides, acetates, tartrates,and the double salts with aluminum called alums. Exemplary of the metalsalts which may therefore be employed are potassium aluminum sulfate,potassium phosphate, cesium aluminum sulfate, rubidium aluminum sulfate,rubidium tartrate, cesium chloride, potassium acetate, potassiumchloride, potassium tartrate and the like. The comparative availabilityof potassium salts renders the salts of this metal particularlypreferred.

Although any of the above metal salts may be employed in the coagulatingbath solution, the use of phosphate salts in particular is preferredinasmuch as the phosphates impart an additional degree of flameresistance to the fiber. Although the flame resistance so attained is ofprimarily a temporary nature, the mere coagulation of a filament in thepresence of phosphate necessarily implies the inclusion of somephosphate within the filament, hence increasing the initial, if not theultimate, flame resistance of the fiber. In any case, it is preferred toemploy such metal salts which have the tendency to improvefiameresistance, rather than employing salts which might tend to havethe opposite effect, such as the nitrates or perchlorates.

In general, the metal salts are present in the solution in amountsranging from about 5 percent by weight based upon the coagulating bathsolution to amounts which completely saturate the bath with salt.However, the absolute amount of salt present in the coagulating bathsolution is not critical. The comparative concentration of fluoboricacid in the polymer dope solution and of metal salt in the coagulatingbath solution will however determine the ultimate amount of fiuoboratefiarneproofing agent which is formed in situ in the fiber. In general,it is preferred that the amount of insoluble fiuoborate salt formed insitu in the fiber range from about 4 to about 10 percent by weight basedupon the ultimate weight of the dried fiber. The

concentration of fluohoric acid in the spinning dope solu tion and ofmetal salts in the coagulating bath can accordingly be adjusted toachieve the desired amount of fluoborate in the ultimate fiber.

Inasmuch as the metal salt is consumed from, the coagulating bath as thefiuoboric acid containing dope solution is fed into the coagulatingbath, it is desirable to continually supplement the concentration ofmetalsalt in the coagulating bath by constant addition of meteredamounts of metal salt. Such a procedure aflords the opportunity toclosely control the fluoborate and to minimize the amount of insolublefluoborate formation outside the fiber. Since fiuoboric acid recoveryfrom the coagulating bath is desirable, avoidance of unnecessaryinsoluble fluoborate precipitation outside the.

fiber should be avoided if possible.

Due to the extremely corrosive nature of fiuoboric acid it is necessaryin conducting the novel process of this invention to thoroughly wash thecoagulated fiber to remove any free fluoboric acid remaining in or onthe fiber. The sparingly soluble fiber are not corrosive but smallamounts of free fiuohoric acid may remain on the fiber after passagethrough the coagulating bath. Accordingly a Washing procedure involving,for example, passing the coagulated fiber through a water cascade atatemperature of about to C.

is subsequently employed. Generally, such a washing, operation isperformed on all acrylonitrile fibers to remove residual solvent afterthe coagulating bath. In conducting the instant novel process suchstandard washing procedures may be adequate to remove the fiuoboricacio,

particularly when the amount of fiuoboric acid in the dope solution hadbeen low. However, lengthen the standard washing operation or provide anadditional washing stepto control the fluoboric acid concentration onthe fiber. Typically the coagulated fiber is subjected to washingconditions as above for a period of 12 seconds to upwards of a minuteeor more.

The novel process of this invention is employed in conjunction with thespinning of acrylonitrile polymers, including homopolymers ofacryonitrile, copolymers of acrylonitrile with copolymerizable monomers,as ,well. as polymer blends, said polymers containing at least about 75percent and preferably at least 85 percent by weight polymerizedacrylonitrile. Other monomeric polymerizable compounds containingethylenic unsaturation which are copolymerizable with acrylonitrile mayform part of the polymer which may be used in the practice of thisinvention. Examples of ethylenically unsaturated monomerscopolymerizable with acrylonitrile include the vacrylates and alkylarcrylates such as methyl acrylate, ethyl acrylate, butyl arcrylate,Z-ethyl hexyl acrylate, methyl concentration in the fiber.

fluoborate salts formed in situ in the it may be necessary to i Example1 To a flask containing 90 grams of a 48 percent by weight solution offluoboric acid there was added 10 grams of an acrylonitrile polymerhaving a composition of 94 percent polymerized acrylonitrile and 6percent of polymerized vinyl acetate. The mixture was heated at 80 to 85C. while stirring. A clear yellow solution resulted. The solution wascoagulated as a filament in water containing 20 percent by weightpotassium aluminum sulfate and percent by weight dimethyl formamide. Theresulting filament was scoured by boiling in fresh water for 5 minutes.The filament obtained was dried in an oven until completely dry. Thefilament could not be ignited and did not support burning upon beingplaced in a flame.

Example 2 The procedure of Example 1 was followed save that thepotassium aluminum sulfate was omitted from the coagulating bath. Thefilament so obtained ignited upon being placed in a flame and continuedto support burning.

Example 3 The procedure of Example 1 was repeated coagulating a mass ofthe same acrylonitrile polymer to form a casting. The coagulating bathin this example consisted of water containing 20 percent by weight oftribasic potassium phosphate. After being washed and dried in an oven asin Example 1 the casting so obtained did not ignite upon being placed ina flame and did not support burning.

Example 4 The procedure of Example 3 was repeated save that the tribasicpotassium phosphate was omitted from the coagulating bath solution.After drying the casting so obtained burned upon being placed in a flameand supported burning upon removal from the flame.

Example 5 To a flask containing 90 grams of a 48 percent by weightsolution of fluoboric acid there was added grams of an acrylonitrilepolymer having a composition of 94 percent by weight polymerizedacrylonitrile and 6 percent by weight of polymerized vinyl acetate. Themixture was heated at 80 to 85 C. with stirring to produce a clearyellow solution. This solution was coagulated as a filament in a Waterbath. The filament so obtained was divided in two parts to provide twosamples. The first sample of the filament was removed from thecoagulating bath and directly dried. The second sample of the filamentwas first rinsed well and then scoured by boiling in fresh water forfive minutes after which it was rinsed and dried.

The first sample which was directly dried did not ignite and did notburn upon being placed in a flame. The second sample ignited upon beingplaced in a flame and supported burning.

Example 6 An acrylonitrile polymer solution prepared as in Example 5 wascoagulated as a filament in 500 cubic centimeters of water containing 20percent by weight aluminum sulphate and containing 25 grams of dimethylformamide. The filament so obtained was divided into two portions toprovide two samples which were treated separately.

The first sample of filament was directly dried while the second sampleof filament was first rinsed. then scoured by being placed in boilingwater for five minutes, after which it was again rinsed and then dried.Upon being placed in a flame neither the filament which was directlydried nor the filament which was subjected to a scouring procedureignited or burned.

Example 7 The procedure of Example 6 was repeated save that thecoagulating solution contained 20 percent by weight of potassiumphosphite and did not contain dimethyl form-amide. A filament wasobtained and divided into two samples to be treated as in Example 6.

It was found that upon being placed in a flame neither the filamentwhich was directly dried nor the filament which was subjected to thescouring procedure ignited or burned.

What is claimed is:

1. A process for uniformly impregnating an acrylic polymer withWater-insoluble fluoborate fireproofing agent comprising the steps of:

(a) dissolving an acrylic polymer in an inert solvent comprisingfluoboric acid, said polymer containing at least by weightacrylonitrile, any balance being another ethylenically unsaturatedmonomer which is copolymerizable with acrylonitrile;

(b) extruding said polymer into a coagulating bath to effect coagulationof said polymer, said bath containing a dissolved metal salt whichreacts with the fluoboric acid of the polymer solution to form aninsoluble metal fluoborate salt in situ within the polymer duringcoagulation thereof, said fluoborate salt being characterized in havinga solubility in water of less than one-half per liter; and

(c) recovering the fluoborate-containing coagulated polymer from thecoagulating bath and removing any free fluoboric acid therefrom.

2. The process of claim 1 wherein said coagulated polymer is in the formof a filament.

3. The process of claim 1 wherein said mer is in the for-m of a film.

4. A process for uniformly impregnating an acrylic polymer withwater-insoluble fluoborate fireproofing agent comprising the steps of(a) dissolving an acrylic polymer in an inert solvent comprisingfluoboric acid, said polymer containing at least about 75 by weightacrylonitrile, any balance being another ethylenically unsaturatedmonomer which is copolymerizable with acrylonitrile;

(b) extruding said polymer solution into an aqueous bath to effectcoagulation of said polymer in the form of a filament, said bathcontaining a dissolved metal salt selected from the group consisting ofpotassium, cesium and rubidium salts which reacts with the fluoboricacid of the polymer solution to form the corresponding fluoborate saltin situ within the coagulated filament; and

(c) recovering the filament from the water bath and removing any freefluoboric acid therefrom.

5. The process of claim 4 wherein the fluoboric acid is present in thepolymer solution in an amount corresponding to at least 5 percent byweight based upon the weight of the acrylic polymer.

6. The process of claim 4 wherein said salt is present in the bath in anamount ranging from about 5 percent to about 25 percent by weight basedupon the weight of the aqueous solution.

7. The process of claim 4 wherein said metal salt is potassiumphosphate.

coagulated poly- 8. The process of claim 4 wherein said metal salt ispotassium aluminum sulphate.

9. The process of claim 4 wherein the acrylic polymer contains about 94%by weight acrylonitrile and the balance of the polymer is vinyl acetate.

10. A fire resistant acrylic film obtained by the process of claim 4.

11. A fire resistant :acrylic filament obtained by the process of claim4.

12. The filament of claim 11 wherein said fluoborate 1 salt is potassiumfluoborate.

8 References Cited FOREIGN PATENTS 560,993 7/1958 Canada.

DONALD E. CZAJA, Primary Examiner.

M. I. WELSH, Assistant Examiner.

